Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocat...

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

Over the past few decades, researchers have established artificial enzymes as highly stable and low-cost alternatives to natural enzymes in a wide range of applications. A variety of materials including cyclodextrins, metal complexes, porphyrins, polymers, dendrimers and biomolecules have been extensively explored to mimic the structures and functions of naturally occurring enzymes. Recently, some nanomaterials have been found to exhibit unexpected enzyme-like activities, and great advances have been made in this area due to the tremendous progress in nano-research and the unique characteristics of nanomaterials. To highlight the progress in the field of nanomaterial-based artificial enzymes (nanozymes), this review discusses various nanomaterials that have been explored to mimic different kinds of enzymes. We cover their kinetics, mechanisms and applications in numerous fields, from biosensing and immunoassays, to stem cell growth and pollutant removal. We also summarize several approaches to tune the activities of nanozymes. Finally, we make comparisons between nanozymes and other catalytic materials (other artificial enzymes, natural enzymes, organic catalysts and nanomaterial-based catalysts) and address the current challenges and future directions (302 references).

Nanomaterials with enzyme-like characteristics (nanozymes): next-generation artificial enzymes

Chemically derived graphene oxide (GO) is an atomically thin sheet of graphite that has traditionally served as a precursor for graphene, but is increasingly attracting chemists for its own characteristics. It is covalently decorated with oxygen-containing functional groups - either on the basal plane or at the edges - so that it contains a mixture of sp(2)- and sp(3)-hybridized carbon atoms. In particular, manipulation of the size, shape and relative fraction of the sp(2)-hybridized domains of GO by reduction chemistry provides opportunities for tailoring its optoelectronic properties. For example, as-synthesized GO is insulating but controlled deoxidation leads to an electrically and optically active material that is transparent and conducting. Furthermore, in contrast to pure graphene, GO is fluorescent over a broad range of wavelengths, owing to its heterogeneous electronic structure. In this Review, we highlight the recent advances in optical properties of chemically derived GO, as well as new physical and biological applications.

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Graphene oxide as a chemically tunable platform for optical applications

Graphene, a two-dimensional, single-layer sheet of sp(2) hybridized carbon atoms, has attracted tremendous attention and research interest, owing to its exceptional physical properties, such as high electronic conductivity, good thermal stability, and excellent mechanical strength. Other forms of graphene-related materials, including graphene oxide, reduced graphene oxide, and exfoliated graphite, have been reliably produced in large scale. The promising properties together with the ease of processibility and functionalization make graphene-based materials ideal candidates for incorporation into a variety of functional materials. Importantly, graphene and its derivatives have been explored in a wide range of applications, such as electronic and photonic devices, clean energy, and sensors. In this review, after a general introduction to graphene and its derivatives, the synthesis, characterization, properties, and applications of graphene-based materials are discussed.

Graphene-Based Materials: Synthesis, Characterization, Properties, and Applications

With advances in exfoliation and synthetic techniques, atomically thin films of semiconducting transition metal dichalcogenides have recently been isolated and characterized. Their two-dimensional structure, coupled with a direct band gap in the visible portion of the electromagnetic spectrum, suggests suitability for digital electronics and optoelectronics. Toward that end, several classes of high-performance devices have been reported along with significant progress in understanding their physical properties. Here, we present a review of the architecture, operating principles, and physics of electronic and optoelectronic devices based on ultrathin transition metal dichalcogenide semiconductors. By critically assessing and comparing the performance of these devices with competing technologies, the merits and shortcomings of this emerging class of electronic materials are identified, thereby providing a roadmap for future development.

Emerging Device Applications for Semiconducting Two-Dimensional Transition Metal Dichalcogenides

Coupling nanomaterials with biomolecular recognition events represents a new direction in nanotechnology toward the development of novel molecular diagnostic tools. Here a graphene oxide (GO)-based multicolor fluorescent DNA nanoprobe that allows rapid, sensitive, and selective detection of DNA targets in homogeneous solution by exploiting interactions between GO and DNA molecules is reported. Because of the extraordinarily high quenching efficiency of GO, the fluorescent ssDNA probe exhibits minimal background fluorescence, while strong emission is observed when it forms a double helix with the specific targets, leading to a high signal-to-background ratio. Importantly, the large planar surface of GO allows simultaneous quenching of multiple DNA probes labeled with different dyes, leading to a multicolor sensor for the detection of multiple DNA targets in the same solution. It is also demonstrated that this GO-based sensing platform is suitable for the detection of a range of analytes when complemented with the use of functional DNA structures.

A Graphene Nanoprobe for Rapid, Sensitive, and Multicolor Fluorescent DNA Analysis

A single-layer MoS2 nanosheet exhibits high fluorescence quenching ability and different affinity toward ssDNA versus dsDNA. As a proof of concept, the MoS2 nanosheet has been successfully used as a sensing platform for the detection of DNA and small molecules.

Single-Layer MoS2-Based Nanoprobes for Homogeneous Detection of Biomolecules

Size and shape of nanoparticles are generally controlled by external influence factors such as reaction temperature, time, precursor, and/or surfactant concentration. Lack of external influence may eventually lead to unregulated growth of nanoparticles and possibly loss of their nanoscale properties. Here we report a gold nanoparticle (AuNPs)-based self-catalyzed and self-limiting system that exploits the glucose oxidase-like catalytic activity of AuNPs. We find that the AuNP-catalyzed glucose oxidation in situ produces hydrogen peroxide (H(2)O(2)) that induces the AuNPs seeded growth in the presence of chloroauric add (HAuCl(4)). This crystal growth of AuNPs is internally regulated via two negative feedback factors, size-dependent activity decrease of AuNPs and product (gluconic acid)-induced surface passivation, leading to a rapidly self-limiting system. Interestingly, the size, shape, and catalytic activities of AuNPs are simultaneously controlled in this system. We expect that it provides a new method for controlled synthesis of novel nanomaterials, design of "smart" self-limiting nanomedicine, as well as in-depth understanding of self-limiting systems in nature.

Self-Catalyzed, Self-Limiting Growth of Glucose Oxidase-Mimicking Gold Nanoparticles

Single-layer transition metal dichalcogenide nanosheets, including MoS2, TiS2, and TaS2, are used as novel sensing platforms for sensitive and selective detection of DNA, based on their high fluorescence-quenching ability and different affinities toward single-stranded DNA and double-stranded DNA. Importantly, for the first time, a single-layer TaS2 nanosheet-based multiplexed DNA sensor is also developed.

Single-layer transition metal dichalcogenide nanosheet-based nanosensors for rapid, sensitive, and multiplexed detection of DNA

Size and shape of nanoparticles are generally controlled by external influence factors such as reactiontemperature,time,precursor,and/orsurfactantconcentration.Lackofexternalinfluencemayeventually lead to unregulated growth of nanoparticles and possibly loss of their nanoscale properties. Here we report a gold nanoparticle (AuNPs)-based self-catalyzed and self-limiting system that exploits the glucose oxidase-like catalytic activity of AuNPs. Wefind that the AuNP-catalyzed glucose oxidationin situproduces hydrogen peroxide (H2O2) that induces the AuNPs' seeded growth in the presence of chloroauric acid (HAuCl4). This crystal growth of AuNPs is internally regulatedviatwo negative feedback factors, size-dependent activity decrease of AuNPs and product (gluconic acid)-induced surface passivation, leading to a rapidly self-limiting system. Interestingly, the size, shape, and catalytic activities of AuNPs are simultaneously controlled in this system. We expect that it provides a new method for controlled synthesis of novel nanomaterials, design of "smart" self-limiting nanomedicine, as well as in-depth understanding of self-limiting systems in nature.

Changfeng Zhu

Papers

A Graphene Nanoprobe for Rapid, Sensitive, and Multicolor Fluorescent DNA Analysis

Single-Layer MoS2-Based Nanoprobes for Homogeneous Detection of Biomolecules

Self-Catalyzed, Self-Limiting Growth of Glucose Oxidase-Mimicking Gold Nanoparticles

Single-layer transition metal dichalcogenide nanosheet-based nanosensors for rapid, sensitive, and multiplexed detection of DNA

Self-Catalyzed, Self-Limiting Growth of Glucose Oxidase-Mimicking Gold